Seasonal and interannual trends in nitrogen and brown tide in Maryland's coastal bays

Abstract

A retrospective analysis revealed that water quality in the coastal bays of Maryland (USA) has been declining over the past decade, as evidenced by increases in total nitrogenous nutrients and in outbreaks of brown tides caused by the pelagophyte Aureococcus anophagefferns. However, the increases in total nitrogen are not a function of increases in inorganic nitrogen, but rather a function of increases in dissolved organic nitrogen (DON). A near-decadal record (1996-2004) demonstrates that an approximate doubling of DON over this time period correlates with a similar increase in total chlorophyll and an even larger increase in the proportion of chlorophyll that is composed of brown tide. Additionally, on an annual basis overall chlorophyll levels and strength of the brown-tide blooms were related to the DON availability that developed during the prior months. Aureococcus anophagefferens is a harmful algal bloom species that preferentially uses DON for its nutrition over inorganic nitrogen forms and thus is a symptom of organic rather than inorganic nitrogen-based eutrophication. These results demonstrate that long-term changes in nutrient quantity and composition have occurred in the coastal bays of Maryland during the past decade and that total phytoplankton biomass, as well as the proliferation of brown tide, are related to these changes. Whether these changes are evidence of a long-term trajectory or represent a short-term anomaly will be revealed in future monitoring. © 2007 by the Ecological Society of America.